Electric hair cutting appliances are generally known and include trimmers, clippers and shavers whether powered by main supplied electricity or batteries. Such devices are generally used to trim body hair, in particular facial and head hair to allow a person to have a well-groomed appearance. These devices can, of course, also be used to trim pet hair or any other type of hair.
Conventional hair cutting devices comprise a main body forming an elongated housing having a front or cutting end and an opposite handle end. A cutting blade assembly is disposed at the cutting end. The cutting blade assembly usually comprises a stationary blade element and a movable blade element which moves in a reciprocal, translatory manner relative to the stationary blade element. The cutting blade assembly itself extends from the cutting end and is usually fixed in a single position relative to the main body of the hair clipper, such that the orientation of the cutting blade assembly is determined by a user orientating the main body of the device.
In common cutting units, the cutting force driving the movable blade is usually transmitted through an electric motor driven eccentric. This eccentric is driven by an electric motor in a rotary manner. The rotary movement of the eccentric is then translated via a so-called driving bridge, which is connected to the movable blade, into the resulting reciprocal, translatory movement of the movable blade.
FIG. 1A schematically illustrates an exemplary arrangement of such a driving bridge 101 on the movable blade 102 as it is commonly realized according to the prior art. As it can be seen from FIG. 1A, the driving bridge 101 according to the prior art is usually mounted or fixed on the upper surface of the movable blade 102. The electric motor driven eccentric usually engages the driving bridge 101 at an engagement point 103, which engagement point 103 is located above the movable cutting blade 102 and has a predetermined distance (indicated as distance h1) from the movable cutting blade 102. The eccentric thus usually has a big distance from the level where the cutting forces from the teeth are working (referred to as cutting level or cutting plane) to the engagement where the electric motor driven eccentric engages the driving bridge. The distance from the engagement of the electric motor driven eccentric to the cutting level in many known prior art hair clipping devices results in a so-called pulling effect. The pulling effect is an unwanted lifting of the movable cutting blade from the stationary cutting blade, which may especially occur during heavy load hair cutting. The reason for this pulling effect is the occurrence of a redoubtable overturning torque that may cause a tilt of the movable blade. The schematical force diagram shown in FIG. 1B visually illustrates the reason for this overturning torque that occurs in most or all known state of the art hair clipping devices.
F1 therein indicates the driving force that is transmitted at the engagement point 103 from the electric motor driven eccentric to the driving bridge 101. F23 indicates the spring force that is usually provided by one or two springs that resiliently bias the movable blade 102 against the stationary blade. When h1 indicates the distance from the electric motor driven eccentric engagement point 103 to the top surface of the movable cutting 102 and e2 indicates the distance between the spring engagement points, then at the moment of lifting (pulling effect) the following relation results:
      F    ⁢    23    =            F      ⁢      1        ·                  h        ⁢        1                    e        ⁢        2            
Under heavy load conditions, e.g. maximum quantity, tightness, length, thickness and/or shape of the hairs, pulling may thus happen during the cutting process in known hair clipping devices. For every home user, professional hair and beard trimmer and also for the hair cutting of pets, the pulling effect is redoubtable as it may generate remarkable hurt by pulling hairs into the device instead of cutting them. Expertise for the above-mentioned pulling effect is known from the applicant's research as well as from other professionals in hair clipping.
In order to overcome this unwanted pulling-effect, two different approaches are generally known. A lot of prior art hair clipping devices try to overcome this effect by applying an enlarged, strong electric motor. However, such an enlarged electric motor is on one hand expensive and on the other hand also voluminous. It thus increases the overall size of the hair clipping device as well as it increases the production costs. Apart from that, the power consumption of such enlarged electric motors is also higher than for hair clipping devices using smaller electric motors. This is especially disadvantages for battery-driven hair clipping devices which in turn have shorter operating times.
The second common approach to overcome the unwanted pulling effect is the usage of a very strong spring, which presses the two cutting blades (movable cutting blade and stationary cutting blade) against each other with a higher force in order to impede a lifting or tilting of the movable blade.
An example of such a cutting unit for hair clipping device is known from DE 103 02 998 A1. Therein, two strong spring elements arranged in a parallel guiding are applied to press the movable blade against the stationary blade. However, such a high pressure between the movable blade and the stationary blade significantly increases the friction between the two blades. This increased friction makes oiling necessary. Besides that, it increases the abrasion of the two blades as well as of the electric motor. This means that also in this solution, large and robust electric motors, which are comparatively expensive and voluminous, need to be applied.